Dataset autodownload feature addition (#685)

* initial commit

* move download scripts into data/scripts

* new check_dataset() function in general.py

* move check_dataset() out of with context

* Update general.py

* DDP update

* Update general.py

data/coco128.yaml

@@ -1,5 +1,4 @@
 # COCO 2017 dataset http://cocodataset.org - first 128 training images
-# Download command:  python -c "from yolov5.utils.google_utils import *; gdrive_download('1n_oKgR81BJtqk75b00eAjdv03qVCQn2f', 'coco128.zip')"
 # Train command: python train.py --data coco128.yaml
 # Default dataset location is next to /yolov5:
 #   /parent_folder
@@ -7,6 +6,9 @@
 #     /yolov5
 
 
+# download command/URL (optional)
+download: https://github.com/ultralytics/yolov5/releases/download/v1.0/coco128.zip
+
 # train and val data as 1) directory: path/images/, 2) file: path/images.txt, or 3) list: [path1/images/, path2/images/]
 train: ../coco128/images/train2017/  # 128 images
 val: ../coco128/images/train2017/  # 128 images